The invention relates to a monitoring circuit for monitoring a voltage, comprising a coil, which is incorporated in a first current branch to be connected to the voltage to be monitored, and an auxiliary relay, the actuating winding of which is incorporated in a second current branch connected in parallel to the first current branch.
The requirement imposed on this monitoring circuit is that the coil, which is also referred to as the zero voltage coil, must be in the de-energised state when the voltage to be monitored has a value which is less than or equal to a predetermined percentage of the nominal value thereof. When the voltage to be monitored increases again the zero voltage coil must be excited again as soon as the voltage to be monitored is at a value which is greater than or equal to a second predetermined percentage of the nominal value thereof
A monitoring circuit of this type having a hysteresis in the switching function is disclosed in laid-open German Patent Application 1 438 960.
The known monitoring circuit is a relay circuit which consists of a first relay in series with a resistor. The DC voltage to be monitored is connected over said series circuit. The known relay circuit also comprises a second current branch parallel to the first current branch in which the first relay and the resistor are incorporated. An auxiliary relay, in series with a make contact, is incorporated in the second current branch. Said make contact is a contact of the first relay. The resistor connected in series with the first relay is bridged by a break contact of the auxiliary relay.
The known circuit is so sized that when the DC voltage to be monitored increases to 5.1 volt sufficient current flows through the actuating winding of the first relay and it is consequently excited. As a consequence of this the make contact that is connected in series with the auxiliary relay is closed, so that the actuating winding of the auxiliary relay is also brought into the excited state. The result of this is that the bridging break contact for the resistor connected in series with the first relay is opened. As a result of opening of said break contact the associated resistor in the actuating circuit of the first relay is switched in and the current in the actuating winding of the first relay is reduced.
The aim of the invention is to provide a monitoring circuit of the type mentioned in the preamble with which the switching hysteresis is reproducible. Moreover, the monitoring circuit must be easily adjustable and of small volume. Furthermore, the monitoring circuit must be robust.
To this end the invention is characterised in that in the first current branch a first make contact of the auxiliary relay is connected in series with the coil, in that in the second current branch a first resistor is connected in series with the actuating winding of the auxiliary relay and in that a series circuit of a second resistor and a second make contact of the auxiliary relay is connected in parallel to the actuating winding of the auxiliary relay.
The invention has the advantage that only one relay is needed in order to obtain any desired hysteresis. When the resistance ratings of the first and second resistor are low compared with those of the actuating winding of the auxiliary relay a more accurate monitoring circuit is obtained which is also not sensitive to changes in temperature.